1. Field of the Invention
The present invention relates to a seat structure including a seat portion and a backrest portion adjustable in inclination angle, and more particularly to a seat structure suitable for nursing equipment.
2. Description of the Background Art
In nursing equipment such as a baby carriage, a baby rack and a child seat, an inclination angle of a backrest portion of a seat is usually adjustable. In the nursing equipment receiving a baby shortly after birth, the backrest portion is inclined backward to an approximately horizontal state in order to realize a bed state. Nursing equipment that can be switched between the form of a bed and the form of a chair in accordance with the developmental stages of infants is commercially available.
FIG. 1 is a view showing a partial seat of nursing equipment in the form of a bed as seen from above. The seat includes a seat portion 1 and a backrest portion 2 adjustable in inclination angle. Seat portion 1 and backrest portion 2 are pivotably coupled to each other such that a flex angle therebetween can be adjusted. A variety of techniques for pivotably coupling these portions may be contemplated, and FIG. 1 shows an exemplary technique.
A back end portion of seat portion 1 has a protruding portion 1a and a notch portion 1b that are positioned alternately. Similarly, a front end portion (or a lower end portion) of backrest portion 2 has a protruding portion 2a and a notch portion 2b that are positioned alternately. A shaft 3 passing through protruding portion 1a of seat portion 1 and protruding portion 2a of backrest portion 2 is provided to allow seat portion 1 and backrest portion 2 to be coupled in a flexible manner.
Usually, a seat plate and a backrest plate made of a resin to form seat portion 1 and backrest portion 2 are provided with concave, groove portions 1c, 2c in places as shown.
FIG. 2 shows a baby lying down on the seat in the form of a bed, and FIG. 3 shows a baby sitting on the seat in the form of a chair. At a flexion portion formed by seat portion 1 and backrest portion 2, that is, a coupling portion therebetween, there exist protruding portions 1a2a and notch portions 1b, 2b through which shaft 3 passes. In the form of a bed shown in FIG. 2, an upper surface of seat portion 1 and an upper surface of backrest portion 2 form an approximately horizontal surface. On the other hand, protruding portions 1a, 2a through which shaft 3 passes projects slightly above the aforementioned horizontal surface, and notch portions 1b, 2b form a concave recessed below the aforementioned horizontal surface.
As shown in FIG. 2, when a baby is lying down, its buttocks are positioned on the coupling portion between seat portion 1 and backrest portion 2. Since the concave and convex formed by protruding portions 1a, 2a, notch portions 1b, 2b, and groove portions 1c, 2c are formed at this portion, a local load acts upon the baby's buttocks in contact with these concave and convex. In other words, in FIG. 2, a body pressure is concentrated on an upper end corner portion B of notch portion 1b and an upper surface C of protruding portion 2a. 
A structure for coupling seat portion 1 and backrest portion 2 to each other in a flexible manner is not limited to that shown in FIG. 1 and a variety of structures may be contemplated. Any adopted structure cannot avoid the formation of concave and convex in the coupling portion between the seat portion and the backrest portion.
In the form of a chair shown in FIG. 3, seat portion 1 and backrest portion 2 are positioned to form a flexion at a prescribed angle. A corner gap 4 as shown is always formed at the back sides of the buttocks of a baby seated on this seat. As a result, a body pressure is concentrated on the baby's buttocks or loins D in contact with the upper end portion of corner gap 4. Medically, sensory nerves giving a feeling of pressure are distributed in the human structures such as the skin, muscles and blood vessels. A pain threshold of sensory nerves varies depending on the respective body region. At present, the pain threshold for a baby's buttocks has not yet been determined clearly, but it is said that the pain threshold at the abdomen is 2.2 Kg/cm2. When the baby's pain sensation is stimulated by the load locally acting upon the baby's buttocks, the baby feels uncomfortable, resulting in an adverse effect on the brain.
In particular, in baby carriages or child seats, the stimulation of the sensory nerves by the local loads is considered great because vibrations involved with movements always act on the seat. Furthermore, in a case of a premature baby, even a small step at a body-supporting portion may have an adverse affect on a baby's growth. Therefore any local loads on the buttocks have to be avoided by any means.
The uncomfortable feeling because of the local loads acting on the buttocks may apply to adult's seats.